Electrical alternans relate to the differences in electrical potential at corresponding points between alternate heartbeats. Twave alternans or alternation is a regular or beat-to-beat variation of the ST-segment or Twave of an electrocardiogram (ECG) which repeats itself every beat group (e.g., two beats, three beats or four beats) and has been linked to underlying cardiac electrical instability. Typically, by enumerating all consecutive heart beats of a patient, beats with an odd number are referred to as “odd beats” and beats with an even number are referred to as “even beats.” A patient's odd and even heartbeats may exhibit different electrical properties of diagnostic significance which can be detected from an ECG.
The ability to detect Twave alternans (TWA) in cardiac signals is important for applications such as monitoring disease progression or risk stratifying patients for arrhythmias. More specifically, the presence of these electrical alternans is significant because patients at increased risk for ventricular arrhythmia's commonly exhibit alternans in the ST-segment and the Twave of their ECG. Clinicians may therefore use these electrical alternans as a noninvasive marker of vulnerability to ventricular tachyarrhythmias. The term Twave alternans is used broadly to denote electrical alternans such as these. It should be understood that the term encompasses both the alternans of the Twave segment and the ST-segment of an ECG or other cardiac signal.
Twave alternans has been demonstrated in many studies as a strong predictor of mortality, independent of left ventricular ejection fraction (LVEF). More specifically, it has become well known that Twave alternans has predictive value for arrhythmic events such as tachyarrhythmias. Additionally, Twave alternans has been determined to be an indicator of various forms of disordered ventricular repolarization, including disorders found in patients with cardiomyopathy, mild to moderate heart failure, and congestive heart failure.
T-wave alternans may be caused by changes in ion exchange during repolarization. If there is an abrupt change in the repolarization period of one beat, the heart attempts to readjust on the following beat. This is manifested as an alternating change in the action potential duration. In the surface ECG this is seen primarily as a change in T-wave. For an implanted medical device such as a cardiac pacemaker, the intracardiac electrogram (IEGM) also shows a change in T-wave. Thus, the term T-wave as used herein may refer to a portion of the ventricular QRS-T-wave complex that includes the T-wave and the QRS-T segment. The alternating feature of Twave alternans can be detected by examination, for example, of the QT interval, T-wave width, T-wave amplitude and morphology, etc.
As discussed in the literature, when such an alternating pattern appears, the different rates or forms of repolarization of the ventricular cells are statistically associated with a variety abnormal cardiac conditions. Further, the alternating repolarization pattern can lead to increased electrical instability and consequent cardiac arrhythmias. Thus, the presence of T-wave alternans is recognized as an indicator of risk for ventricular arrhythmia and even sudden cardiac death (SCD).
Cardiac signals can suffer from noise artifacts. Additionally, the presence of Twave alternans can be inherently hard to detect because of their small signal amplitudes. Accordingly, there is a need to provide for proper Twave alternans detection that relies on accurate distinction between noise artifacts and Twave alternans.
Whatever the designated portion of the intracardiac electrogram, electrical alternans refers to an alternating pattern of the wave that can be designated “ABABAB . . . ” where A represents every other cycle and B represents every other alternate cycle. Such a pattern is often referred to as a two beat alternans pattern, or simply an AB pattern. Electrical alternans may also refer to an alternating pattern of the wave that can be designated “ABCABC . . . ”, or an alternating pattern of the wave that can be designated “ABCDABCD . . . ”. The “ABCABC . . . ” pattern is a three beat alternans pattern, which can be simply referred to as an ABC pattern, and the “ABCDABCD . . . ” pattern is a four beat alternans pattern, which can be referred to as an ABCD pattern.
While ABC, ABCD, and longer alternans patterns have been visually observed in ECG signals, conventional Twave alternans detection algorithms are typically designed to search for the two beat alternans patterns (i.e., AB patterns). However, as mentioned above, a person may experience other types of alternans patterns, such as three beat patterns (i.e., ABC patterns), four beat patterns (i.e., ABCD patterns), etc. If an algorithm is searching for an AB pattern, but the person is experiencing an ABC pattern, the algorithm may not detect the presence of alternans. In other words, the algorithm may produce a false negative. It would be beneficial if such false negatives could be minimized.
It has been generally believed that an elevated constant heart rate is a requirement for the detection of Twave alternans. However, a recent work published by Bullinga et al., entitled “Resonant Pacing Improves Twave Alternans Testing in Patients with Dilated Cardiomyopathy” (Heart Rhythm v1:S129, 2004) suggested a more robust detection with “resonant pacing” scheme. In this technique, Twave alternans with higher amplitudes were detected by pacing at a relatively shorter interval periodically once every fourth cycle during a moderately fast and constant pacing routine. Additionally, U.S. patent application Ser. No. 10/884,276 to Bullinga, filed Jul. 2, 2004 (Pub. No.: US 2005/0004608 A1), which is incorporated herein by reference, suggested that various types of patterned pacing sequences can be used to induce oscillations of Twaves. When inducing oscillations of Twaves, the type of alternans pattern being induced is controllable, and thus known. It would be beneficial to provide algorithms that can search for expected alternans patterns, other than the typical searched for AB patterns.